There is an increasing interest in energy efficient lighting to replace conventional incandescent bulbs, not least because of environmental concerns. Whereas compact fluorescent lamps (CFL) presently dominate energy efficient lighting, there is an increasing move towards light emitting diode (LED) lighting, since this offers the prospect of a significant reduction in energy consumption, with respect even to CFL.
However, in common with CFL, LED lighting typically takes the form of a high ohmic load. This presents challenges for existing lighting circuits incorporating a dimmer circuit: the most common types of dimmer circuits are phase cut dimmers, in which the mains supply is cut off for part of the mains cycle—either the leading edge of the cycle or half-cycle, or its trailing edge. Most trailing edge dimmers are based on a transistor circuit, whereas most leading edge dimmers are based on a triac circuit. Both transistor and triac dimmers require to see a low ohmic load.
To satisfy this requirement, it is known to provide LED driver circuits (also known as electronic ballasts), with a “bleeder”, which presents a relatively low ohmic load to the dimmer circuit in order to ensure that it operates correctly. However, if the circuit including bleeder is connected to a non-dimmable mains connection, the bleeder operates unnecessarily, resulting in an efficiency drop, which typically can be up to 10%, and potentially increased electromagnetic interference (EMI) problems if the bleeder is dynamically controlled.
An LED driver circuit is known in which the bleeder may be disconnected in the absence of a dimmer circuit. Such a circuit is disclosed for instance in United Kingdom Patent Application publication GB-A-24357264.